1. Field of the Invention
The present invention relates to a diode-split high-voltage transformer having a core, a primary winding and a high-voltage winding, which is arranged in chambers of a coil former.
2. Description of the Related Art
A diode-split high-voltage transformer of this type is disclosed in EP 0 529 418 B1, for example. This transformer contains a first coil former, which accommodates the primary winding and further auxiliary windings, and a second coil former, in which is arranged the high-voltage winding in the form of a chamber winding. The two coil formers are usually produced and wound separately. During final assembly, the coil former with the high-voltage winding, which has a correspondingly larger inner diameter, is pushed over the coil former with the primary winding. The coil formers are subsequently surrounded with a plastic housing and additionally potted with a synthetic resin composition for the purpose of suppressing corona effects and high-voltage flashovers. Embodiments of this type are used in television sets, for example, and supply high voltages in continuous operation of 24 kV up to above 30 kV.
DE 38 22 284 A1 discloses a high-voltage transformer having small dimensions for about 7 kV for copiers and the like. This transformer likewise has two coil formers, the coil former with the primary winding being pushed over the coil former with the high-voltage winding and latching into place therein. It is not designed as a diode-split high-voltage transformer and it cannot achieve high voltages of above 20 kV as required for television sets. It contains no rectifier diodesxe2x80x94these are arranged separately in the associated circuit. The particular intention, by using a chamber-type coil former, was to solve the high-voltage problems which arise here due to the small distance between the high-voltage winding and the core. Despite the considerably lower voltage of 7 kV, however, this design has not demonstrated satisfactory high-voltage strength in sustained operation, even with complete potting, and has therefore not been put into production.
The object of the present invention is to specify a diode-split high-voltage transformer of the type mentioned in the introduction which is constructed very compactly and cost-effectively and, in particular, has good high-voltage strength in continuous operation at voltages of above 20 kV.
This object is achieved by means of the invention specified herein. Advantageous developments of the invention are specified in the foregoing description.
In the case of the diode-split high-voltage transformer of the invention, the primary winding lies above the high-voltage winding and the high-voltage transformer contains means by which the electric field between the coil former and the core is reduced in order to avoid corona effects. For example, the surface of the inner cavity of the coil former is provided with a conductive coating, which, during operation, is at earth as a result of contact with the core, or at the same potential as the core. As a result, the electric field can be screened in the inherently unavoidable air gap between core and coil former, thereby effectively suppressing corona effects and voltage flashovers. Corona effects are produced in particular by ozone produced in the air by a high electric field. The conductive coating concentrates the electric field in the material between the high voltage winding and the conductive coating of the coil former, which ensures long-term high-voltage strength with an appropriate material and dimensioning.
The conductive coating used must be a high-impedance layer, for example colloidal graphite, which can be applied in a simple manner by means of a nozzle which sprays in the radial direction. A low-impedance, for example metallic, layer would constitute a short-circuited turn and lead to losses.
As an alternative, instead of the conductive coating, the remaining cavity between core and coil former may be filled with a material, so that corona effects are also avoided by this means. The material preferably has the highest possible relative permittivity ∈r, for example 2-3 or 4, and may be, for example, a viscous paste, possibly also the potting material of the high-voltage transformer itself. The material may also have a low conductivity. Air inclusions must not occur in the course of filling since, on account of the low relative permittivity ∈r=1, a high electric voltage builds up in the said inclusions and air can easily be ionized under the voltage conditions prevailing here.
Since the primary winding bears together with an insulating layer directly on the high-voltage winding, the entire arrangement becomes very compact. The chambers of the coil former also provide, with a multiple sheet winding, a sufficiently smooth surface onto which the primary winding can be wound uniformly and tightly with a wire thickness of, for example, 0.3 to 0.8 mm.
The wall thickness under the chambers of the high-voltage winding in the direction of the core are advantageously chosen such that they increase as the high voltage rises at the bottom of the chamber.
The high-voltage diodes may be arranged laterally with respect to the high-voltage chambers on the coil former, or alternatively they may be integrated between the high-voltage winding and the primary winding. In order to obtain a very inexpensive embodiment, the high-voltage winding is subdivided into four windings, a diode being respectively connected between the first and the second and the third and the fourth winding and a tap being routed out between the second and third winding for the focus voltage of a picture tube.
The compact structure of the coil former enables not only the housing of the high-voltage transformer but also its core to be considerably reduced in size. As a result of this, the potting compound can also be considerably reduced since there are no longer any high-voltage potentials on the outside of the high-voltage transformer. This not only leads to a considerable cost reduction but also affords space and weight advantages. Thus, it is possible to achieve a weight reduction of 25%, given the same electrical properties, with a diode-split high-voltage transformer (DST) having two diodes compared with a diode-split high-voltage transformer having three diodes. In addition, RLC circuits for attenuating the interference radiation are obviated.
In a further exemplary embodiment, the diode-split high-voltage transformer contains only one coil former, in which the high-voltage winding is arranged in chambers, the primary winding lying above the high-voltage winding and being wound onto an interposed sleeve or sheet winding. As an alternative, it is also possible to use a simple coil former for the primary winding, which coil former is pushed over the coil former with the high-voltage winding. If a sleeve is used, it may also be composed of two or more parts.
In an advantageous manner, the primary winding is somewhat wider than the high-voltage winding and covers the latter as far as possible completely. The higher-frequency interference radiation produced in the high-voltage winding is virtually completely screened by this means since the core (usually at earth potential) of the high-voltage transformer is situated on the inside of the high-voltage transformer and the covering, tightly wound primary winding is situated on the outside, and the outer chambers of the high-voltage winding carry either no or only a very small pulse voltage, depending on the design, since they are connected either to the reference potential or to the high-voltage connection directly or via a further chamber. These interference voltages are produced as a result of oscillations between the inductances and stray capacitances of the high-voltage transformer when the diodes change over from the conducting phase to the blocking phase. These facts have already been explained comprehensively in the literature, for example in EP 0 735 552 A1, and are not, therefore, discussed in any further detail here.
Since the primary winding is advantageously situated such that it fits above the high-voltage winding, the diodes cannot be arranged directly between the corresponding partial windings, for example on the webs of the chambers or above the chambers, rather they have to lie outside. The connections of the diodes to the high-voltage chambers are in this case routed via interposed high-voltage chambers. Very good coupling between the high-voltage winding and the primary winding is achieved, moreover, by the compact arrangement of the high-voltage transformer.
It is possible to arrange up to two diodes in a chamber in the lower part of the coil former lying in the direction of the circuit board. On the upper side of the coil former, diodes may be arranged on a continuation of the coil former. In particular, the lower diodes are arranged parallel to the lower part of the lower core limb and the upper diodes are arranged perpendicular to the upper part of the upper core limb, with the result that it is possible to use a core whose clear width is only slightly larger than the length of the primary and high-voltage windings since in this case the said core can be passed laterally out of the coil former through cutouts. The upper diodes are additionally arranged in such a way that after the winding of the high-voltage winding and the mounting and connection of the diodes, a single-part sleeve fitting exactly over the high-voltage winding can be pushed over the diode and the high-voltage winding.
Arrangements of diodes between high-voltage winding and primary winding are likewise possible, however. These may lie for example axially with respect to the coil former above the high-voltage chambers, parallel to the core, with the result that connections between partial windings of the high-voltage winding are simultaneously established hereby. The periphery of the primary winding consequently becomes slightly larger and may also acquire an oval shape.
The use of a larger number of diodes is also possible as the means for reducing the electric field for the purpose of avoiding corona effects. In further developments it has surprisingly been found that a high-voltage transformer of this type operates reliably even without a conductive coating. Thus, for example, a high voltage of 32 kV can be reliably generated in sustained operation with four diodes. It is still possible to obtain up to about 28 kV with three diodes, but this represents an uncertain upper limit. In a type with three diodes, therefore, a conductive coating is recommendable since the latter can be applied in one work operation with virtually no additional costs.
The explanation for the sufficient high-voltage strength for high-voltage transformers having three or more diodes without a conductive coating is that the outer chambers carry virtually no pulse voltages and in the inner chambers, by virtue of the larger number of diodes, the pulse voltages do not reach a voltage value which might lead to corona effects between the high-voltage chambers and the core.
The high-voltage transformer can be produced cost-effectively since it has only one complicated plastic component, the coil former with the high-voltage winding. Since the thin wire of the high-voltage winding, typically about 0.05 mm, is wound first in this case, this winding operation can be controlled very well. The sleeve or a sheet winding is subsequently applied and the thick wire of the primary winding and any further auxiliary windings can be wound on it without any problems. Since, in this arrangement, virtually no high voltage-carrying parts, in particular no parts having pulse voltages, lie on the outside of the coil former, and thus on the outer edge of the high-voltage transformer, the thickness of the synthetic resin composition between the coil former with the windings and the outer plastic housing of the high-voltage transformer can be reduced from 3 mm to less than 1 mm, as a result of which the plastic housing can be considerably reduced in size.
Since the primary winding now lies outside the high-voltage winding rather than within the latter, it is comparatively remote from the stray fields of the core, which are highly pronounced particularly around the air gap. Since the interference oscillations contain higher harmonics up to above 1 MHz, pronounced losses arose previously in the primary winding due to skin effects and eddy currents, which could be kept to a tolerable level only by means of thin wires of the primary winding, in particular by using expensive multiple-stranded wire. The novel arrangement makes it possible to use thick wire, for example copper having a thickness of 0.475 mm or more, without pronounced skin losses arising, as a result of which it is also possible to reduce the resistive losses in the primary winding. However, the primary winding situated on the outside must absorb the emitted interference radiation. In a preferred exemplary embodiment, the primary winding lies at a distance of about 7 mm from the core, whereas in earlier designs the distance is typically 1.5 mm.
The smaller periphery of the high-voltage winding means that the winding capacitances are considerably lower. This enables the number of turns to be increased, as a result of which the diameter of the ferrite core could be reduced. This not only affords a cost saving and space saving but also the losses in the ferrite core are reduced.
Further advantages are safe operation since, in the event of a short circuit in the high-voltage winding, which may lead to overheating, the transformer can no longer burst open because the high-voltage winding is surrounded very solidly by the primary winding which is wound tightly with thick wire. Furthermore, there is no need for an RLC circuit connected to the primary winding since the high-voltage is sufficiently stable. A design having four diodes enables, for example, a high-voltage transformer with 60 watts output voltage on the high-voltage side at 32 kV which has a cost reduction of more than 20% and is about the same size as a previous 30 or 40 watt transformer, with a weight of 200 grams. The weight can be reduced overall by 30% compared with earlier types having the same power output. Moreover, the height of the high-voltage transformer can be kept very low since the high voltage can be routed out at the bottom of the chambers and passed via a plastic sleeve in the housing from bottom to top to the connection. Insulation necessitates a tube of about 4 cm, virtually all of which lies in the housing of the high-voltage transformer. The present high-voltage transformer is thus excellently suited to recent television set or monitor chassis since the chassis structure is becoming ever more compact as a result of integrated circuits having higher and higher levels of integration. It need no longer be feared that interference radiation will interfere with the tuner circuit.